Follow the author of this article

Follow the topics within this article

Britain is dealing with an unprecedented terrorist threat which has seen MI5 and police disrupt five terror plots in the past two months alone, a senior Whitehall source has said.

The threat from Islamist terrorists intent on committing attacks in the UK is so high that the security services are currently running 500 active investigations looking at some 3,000 potential suspects.

Counter terrorism officials on Thursday sought to disclose the scale of the menace as MI5 and police faced accusations they had missed chances to stop the Manchester bomber when he was repeatedly flagged to authorities as a danger.

Armed police are patrolling trains for the first time after the terror threat level was raisedCredit:
PA

As the country remained on its highest terror alert for a decade, it was announced armed transport police would patrol trains for the first time.

With the public urged to be vigilant, police were called to a string of incidents that turned out to be false alarms. Bomb squad officers were called to a school in Manchester, while Westminster Bridge and a shopping centre in Newport were both closed over fears of a suspicious car and Swansea Magistrates court was evacuated over a suspect package.

The threat from battle-hardened jihadists returning from Iraq and Syria and the peril of online radicalisation are contributing to highest threat seen in decades by MI5.

A total of 18 plots have been wound up since 2013, including five in the two months since Khalid Masood killed four people during a car and knife rampage in Westminster.

The source said MI5 was managing around 500 active investigations, involving some 3,000 subjects of interest at any one time.

The source said: “Abedi was one of a larger pool of former subjects of interest whose risk remained subject to review by MI5 and its partners.

The source said where former subjects of interest seemed to show a risk of heading back into terrorism “MI5 can consider re-opening the investigation, but this process inevitably relies on difficult professional judgements based on partial information.”

A terror attack in the UK is expected imminently after the threat level was raised to critical in the wake of Tuesday’s attack. The question of whether counter terrorism forces have enough funding or resources for the fight against terrorism is now likely to become a General Election issue.

One former senior security figure said: "Knowing of someone's radical sympathies and knowing they present a real and present danger are very different things.

"So the essence of the security dilemma is triage, how to assess who and when to investigate very deeply given the resources needed for 24/7 surveillance.

"For every suspect that appears to be high priority another has to be pushed down the list.

"So who not to investigate urgently is as important a decision as who might be worth investigating."

Shashank Joshi, senior research fellow at security think tank the Royal United Services Institute, said: "It's easy, with the benefit of hindsight, to argue that these warnings were opportunities to stop the bomber.

"However, it's also possible that these warnings were followed up, surveillance was conducted, and nothing was discovered.